Wavelet coherence of surface pressure fluctuations due to von Kármán vortex shedding near a hemispherical protuberance
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Wavelet coherence analyses are done using fluctuating surface-pressure data, to reveal interesting aspects of von Kármán (vK) vortex shedding from a hydraulically smooth hemispherical protuberance. This study is conducted at a hemisphere-diameter (D) based Reynolds number of \(Re_D = 6.36 \times 10^4\), and for an oncoming turbulent boundary layer with thickness 0.16D. Benefits of wavelet analysis are emphasised for these classes of flows. Aided with other published results where possible, it is found in this present study that: (1) span-wise coherency of vK vortex formation is aperiodic, with time intervals well approximated by a Markov process; (2) the predominant appearance of vK shedding for present conditions, as opposed to symmetric shedding, suggests an influence of the oncoming boundary-layer thickness and hence local turbulence intensity on shedding type; and (3) vK vortex convection velocity in the hemisphere near wake approximately collapses onto a power law as a function of stream-wise distance, with vortex incoherency occurring beyond two hemisphere diameters downstream.
The authors wish to thank their DST colleagues, whose advice and technical assistance are appreciated. The authors also wish to express their gratitude to the anonymous reviewers, whose comments helped shape the scope and content of the study. Some MATLAB code used here for wavelet analyses was sourced from Torrence and Compo (1998) and Grinsted et al. (2004), and modified for the author’s specific study.
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